The invention relates to electric discharge lamps of the high pressure metal vapor type and is especially applicable to such lamps having a metallic halide fill.
High pressure metal vapor arc discharge lamps generally comprise an elongated arc tube made of quartz or fused silica and having pinches or press seals at each end. The arc tube contains a quantity of mercury along with an inert starting gas such as argon and is provided with electrodes at opposite ends supported by the press seals. Metallic halide lamps contain in addition to the mercury and starting gas one or more metal halides such as sodium, thallium, and indium iodides, or sodium and scandium iodides. In commercial lamps, the arc tube is generally enclosed within a vitreous outer envelope or jacket provided with a screw base at one end.
Arc tubes are now commonly made utilizing so-called full press seals wherein the entire end segment of a piece of quartz or fused silica tubing is collapsed and sealed off. This is done by pinching the ends of the quartz tube while in a heat-softened condition between a pair of opposed jaws to press the quartz about a foliated inlead supporting an electrode on its inner end. The jaws contact and compress only the end portions of the quartz tubes which form the press seals or pinches about the inleads. The immediately adjacent quartz which is viscous at the instant of pinching assumes a generally rounded shape in the transition zone between the cylindrical main body of the arc tube and the press seal which may be referred to as the end chamber. The shape or blow-out of the end chambers, that is of the space around and behind the electrodes, will vary with the type of quartz, the wall thickness, the heat concentration and the nitrogen pressure build-up at pressing.
With the conventional high pressure mercury vapor lamp of long standing, the specific shape of the end chambers is not particularly significant and there has been little concern over variations from lamp to lamp. Because such lamps operate with the mercury all vaporized, the metal vapor density is substantially independent of envelope temperature, and end chamber variations do not appreciably affect performance or electrical characteristics.